1. Field of Invention
The invention relates to a microelectroforming mold using a preformed metal as its substrate and the fabrication method of the same. More particularly, this invention relates to a microelectroforming mold that uses a bonding material to combine a metal substrate and a metal microstructure, forming a preformed metal substrate, and the fabrication method of the same.
2. Related Art
With the advance in technologies, miniaturization is the trend in the microelectronic industry. Microengineering techniques are thus developed to perform mass production to lower the manufacturing costs for microelectronic devices. Developed in Germany, the LIGA (which is called Lithographie, Galvanoformung, and Abformung, meaning synchrotron radiation lithography, galvano forming, and plastic molding) procedure integrates deep X-ray lithography, micro-electroforming, micro-injection and micro-embossing in the micro engineering techniques is an optimal method for mass-producing high aspect ratio and high precision microengineered components among others. The standard LIGA procedure uses high-energy synchrotron radiation as its lithography X-ray source to make sub-millimeter microstructures. There is a wide selection of materials that this procedure can apply to. For example, it can be used to manufacture metal and plastic microstructures. With these advantages, LIGA is recognized as the best technique for fabricating high aspect ratio and high precision 2D and 3D microengineered components. However, this technique has the problems of high tooling costs, a complicated procedure, and a long manufacturing time, LIGA-like procedures using UV light, laser or plasma as the light source has become another growing trend in the field.
LIGA-like procedures using SU-8 negative type photoresist can employ UV light to get high aspect ratio microstructures. The method using UV light as the lithography light source is called the UV-LIGA procedure. With the use of the electroforming technique, a high aspect ratio microengineered electroforming mold can be made. Taking the microengineered electroforming mold and directly applying it to plastic injection and micro-embossing can mass-produce low-cost microengineered elements. In addition, most of current electroforming techniques used in the UV-LIGA procedure use silicon wafers as the substrate. The silicon wafer surface is formed with a metal thin film (called a seed layer) by vapor deposition to make it conductive. Photoresist is then applied on the silicon wafer surface to define a microstructure. The silicon wafer surface and the photoresist surface are electroformed to duplicate the micro structure. After the silicon wafer and the photoresist are removed, one finally performs machining to adjust the size, obtaining an electroformed mold. The microelectroforming technique mentioned above has to electroform a metal to a thickness of the millimeter order so as to achieve the bulk strength required by the microelectroforming mold. The residual stress in the electroformed metal increases with the thickness of the electroformed metal. Therefore, the microelectroforming mold is likely to deform in shape. In addition, subsequent machining will also produce stress in the material to deform the mold. This deformation is very hard to control in current technology. Moreover, the SU-8 negative type photoresist becomes cured resin after the exposure that is difficult to be removed from the electroforming mold, affecting subsequent plastic injection and micro-embossing. The precision control of the microengineered components thus becomes the bottleneck of the UV-LIGA procedure, greatly restricting the development of this technique.
To avoid the deformation problem in the electroforming molds, one has to remove the residual stress in electroforming. The residual stress in turn depends upon many factors, such as the composition, pH value, temperature, additives, heavy metal impurities of the plating solution. These parameters can be controlled in the electroforming procedure in practice. For instance, using proper additives can reduce the residual stress. However, many other factors may change local current densities. For example, changes in geometrical shapes of the microstructures, designs of the plating bath, electroforming mold structure and material will all change the local current densities, resulting in residual stress that deforms the electroforming mold structure. Another method of reducing the residual stress in electro forming molds is to use a material the same as the electroforming metal (such as nickel) as the substrate instead of using the silicon wafer. Nevertheless, this metal substrate manufacturing procedure renders only a machine bonding strength between the electroforming mold microstructure and the metal substrate. The bonding is so bad that it does not meet the strength and multiple use requirements for a mold.